1. Manipulating Pictures, Arrays, and Loops part 6
Barb Ericson
Georgia Institute of Technology
August 2005
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Learning Goals
Understand at a conceptual and practical level
How to change all the colors in a method
How to use a for loop
How to negate a picture
How to turn a picture into grayscale
How to adjust the grayscale based on how we perceive colors
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Negating an Image
How would you turn a picture into a negative?
White should become black
255,255,255 becomes 0,0,0
Black should become white
0,0,0 becomes 255,255,255
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Negate Algorithm
Subtract current value from 255 for red, green, and blue
Get the array of pixels from the picture
Declare variables to hold the current pixel and the red, green, and blue values
Loop starting an index at 0 and incrementing by 1 and loop while the index is less than the length of the array
Get the pixel at the current index from the array of pixels
Set the red value to 255 – current red value
Set the blue value to 255 – current blue value
Set the green value to 255 – current green value
Increment the index and go back to step 3
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Negate Method
/**
* Method to negate the picture */
public void negate() {
Pixel[] pixelArray = this.getPixels();
Pixel pixelObj = null;
int redValue, blueValue, greenValue = 0;
// loop through all the pixels
for (int i = 0; i < pixelArray.length; i++) {
// get the current pixel
pixelObj = pixelArray[i];
// get the values
redValue = pixelObj.getRed();
greenValue =
pixelObj.getGreen();
blueValue = pixelObj.getBlue();
// set the pixel's color pixelObj.setColor(
new Color(255 - redValue,
255 - greenValue,
255 - blueValue)); }
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Changing to Grayscale
Grayscale ranges from black to white
The red, green, and blue values are the same
How can we change any color to gray?
What number can we use for all three values?
The intensity of the color
We can average the colors
(red + green + blue) / 3
Example
( ) / 3 = 90
To average x numbers get the sum of all the numbers and then divide by x.
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Grayscale Algorithm
Set color values to the average of the original values
Get the array of pixels from the picture
Declare variables to hold the current pixel and the red, green, and blue values
Loop starting an index at 0 and incrementing by 1 and loop while the index is less than the length of the array
Get the pixel at the current index from the array of pixels
Calculate the average of the current values
(redValue + greenValue + blueValue )/ 3
Set the red value to the average
Set the blue value to the average
Set the green value to the average
Increment the index and go to step 3
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Grayscale Method
/**
* Method to change the picture to gray scale */
public void grayscale() {
Pixel[] pixelArray = this.getPixels();
Pixel pixelObj = null;
int intensity = 0;
// loop through all the pixels
for (int i = 0; i < pixelArray.length; i++) {
// get the current pixel
pixelObj = pixelArray[i];
// compute the average intensity
intensity =
(pixelObj.getRed() +
pixelObj.getGreen() +
pixelObj.getBlue()) / 3;
// set the pixel color
pixelObj.setColor(new Color(intensity,intensity,intensity)); }
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Testing Grayscale
String file =
“c:/intro-prog-java/mediasources/caterpillar.jpg”;
Picture pictureObj = new Picture(file);
pictureObj.explore();
pictureObj.grayscale();
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Grayscale Result
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Luminance
We perceive blue to be darker than red, and green
Even when the same amount of light is reflected
A better grayscale model should take this into account
Weight green the highest (* 0.587)
red less (* 0.299) and
blue the very least (* 0.114)
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12. Grayscale with Luminance Exercise
Create a new method grayscaleWithLuminance
Using the new algorithm for calculating intensity
intensity = (int) (red * green * blue * 0.114)
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13. Testing Grayscale with Luminance
String file =
“c:/intro-prog-java/mediasources/caterpillar.jpg”;
Picture pictureObj = new Picture(file);
pictureObj.explore();
pictureObj.grayscaleWithLuminance();
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Summary
You can change all the colors in a method
By creating a new color object
You can negate a picture
By creating a new color with (255 - red, green, blue)
You can create a grayscale picture
By setting the red, green, and blue to the same value
You can improve this by weighting the colors based on how we perceive them
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